四万十川河口北方付近の海成段丘堆積物

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Middle marine terrace deposits near the northern mouth of the Shimanto river, southwestern part of Kochi Prefecture are summarized as follows; 1. Middle marine terrace about 30-35m in altitude is developed along the western coast of Tosa Bay in the surveyed area. 2. This consists of palechtalus, marine deposits, gravel and sand in descending order. 3. Paleotalus deposit which has about 10m thick covers unconformably the basement rocks of the Shimanto terrain. This always exits below the marine deposits, and is also observed near Yamada, about 20km west of Hirano. 4. Marine deposits, 10-20m in altitude, which are mainly composed of mud and partly sand have 60cm thick, and the uppermost part becomes always orange silt. Many fossils such as mollusca, crab, coral, foraminifer and pollen are found in this marine deposits. 5. Gravel bed mainly composed of beach gravel covers the marine deposits, and at its lower part is intercalated partly with gravel and sand. In the matrix of these gravel beds there are yellow tuffaceous matters, which show augite type (Fig. 7) and contain hydrated halloysite and gibbsite at some horizons. These yellow tuffaceous matters show quite different type among Quaternary tephra of Kyushu and Shikoku. 6. Sand bed covers the gravel bed, and becomes reddish brown at the upper part. This sand is widely distributed in the surveyed area. 7. As the results of pollen analysis from Loc. 2 and 5, two elements, warm temperate and north one, are found: the former; Podocarpus, Cyclobalanopsis, Celtis, Trema and Lagerstroemia, the latter; Fagus, Stewartia, Ulmus and Juglans. The plants of these two elements formed the mixed vegetation, which are not observed nowadays. From this fact these deposits may be correlated as Mindel-Riss interglacial stage 8. These marine deposits may be correlated as Nagasu Formation of Kyushu.
日本地質学会の論文
1972-12-25

四万十川河口北方付近の海成段丘堆積物

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